1. Field of the Invention
The present invention relates to a two-stage hydrotreating process in which a heavy hydrocarbon feed is contacted in a first hydrotreating zone with hydrogen and a relatively small pore size catalyst and the hydrotreated hydrocarbon is subsequently contacted with hydrogen in a second hydrotreating zone with hydrogen and a larger pore size catalyst having a specific pore size distribution. More particularly, the invention relates to a two-stage hydrodesulfurization and hydrodemetallization process utilizing a different catalyst in each stage.
2. Description of the Prior Art
Hydrodesulfurization processes in which heavy hydrocarbon distillates or residual fractions are hydrotreated with hydrogen in the presence of a catalyst comprising a hydrogenation component composited with a refractory oxide support, such as alumina, are well known (see for example, U.S. Pat. Nos. 3,531,389; 3,509,044 and 3,770,618).
When it is desired to decrease the sulfur content of high sulfur-containing heavy hydrocarbon feed to a low level, such as, for example to 0.3 weight percent sulfur, large amounts of the conventional catalysts must be utilized. Atlernatively, a high treating temperature would favor desulfurization, however, when a heavy hydrocarbon oil containing large amounts of asphaltenes, metal contaminants and sulfur is used as feed, high start of process run temperatures are usually avoided because they favor the formation of carbonaceous material which deposits on the catalyst and which thereby rapidly decreases the catalytic activity.
Multi-stage processes of hydroprocessing have been proposed in an attempt to overcome some of the disadvantages of the one-stage processes. For example, U.S. Pat. No. 3,766.058 teaches a two-stage process for hydroprocessing a heavy hydrocarbon feedstock in which the second stage catalyst has a larger pore diameter than the first stage catalyst. U.S. Pat. No. 3,876,530 teaches a multi-stage catalytic hydrodesulfurization and hydrodemetallization of residual petroleum oil in which the initial stage catalyst has a relatively low proportion of hydrogenation metals and in which the final stage catalyst has a relatively high proportion of hydrogenation metals.
U.S. Pat. Nos. 3,876,523 and 3,891,541 teach processes for the hydrodemetallization and hydrodesulfurization of residual oils utilizing a catalyst having specific pore size distributions.
U.S. Pat. No. 3,907,668 teaches hydrodesulfurization of a sulfur-containing asphaltene-free petroleum distillate with a catalyst having a specific pore size distribution.
It has now been found that special advantages can be obtained by subjecting a heavy hydrocarbon oil to a two-stage hydrotreating process in which the second stage catalyst, which has a larger median pore radius than the first stage catalyst, has a critical pore size distribution.